ThermalMode: `Supported` property from const to readonly

Update the `Supported` property from const to readonly according to the
updated definition of these flags.

Change-Id: Icece66fd3f4528e744d73a45d611796b993ae1e4
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/xyz/openbmc_project/Control/ThermalMode.interface.yaml b/xyz/openbmc_project/Control/ThermalMode.interface.yaml
index b448d69..742ba35 100644
--- a/xyz/openbmc_project/Control/ThermalMode.interface.yaml
+++ b/xyz/openbmc_project/Control/ThermalMode.interface.yaml
@@ -12,7 +12,7 @@
     - name: Supported
       type: array[string]
       flags:
-          - const
+          - readonly
       description: >
           An implemention specific list of supported modes that the thermal
           control application can be configured to provide for a platform.